1. Technical Field
The present invention relates to a wireless transmission system and a wireless transmission method in which data is exchanged between a plurality of wireless transmission units, and to a wireless station and a transmitting station for use therein. More particularly, the present invention relates to a wireless transmission system and a wireless transmission method in which a plurality of wireless transmission units transmit signals using an anti-multipath transmission scheme, and to a wireless station and a transmitting station for use therein.
2. Background Art
In the field of wireless communication, there is a technique using an anti-multipath modulation/demodulation scheme, in which a signal is transmitted simultaneously from a plurality of transmitting stations to deliberately create a plurality of signal paths, and a plurality of arriving signals are combined together on the receiver side, thus obtaining a path diversity effect and thereby improving the transmission characteristics.
For example, modulation/demodulation schemes with an anti-multipath property include those in which an improvement is made to the modulation scheme, including a spread spectrum scheme, an orthogonal frequency division multiplexing (OFDM) scheme in which information is transmitted while being distributed among a large number of subcarriers arranged over a wide frequency range, and a so-called “anti-multipath modulation scheme” in which an anti-multipath property is exerted by providing a phase or amplitude redundancy in the transmitted symbols, e.g., a PSK-VP (phase shift keying with varied phase) scheme (Non-Patent Document 1) in which a convex-shaped phase redundancy is provided or a PSK-RZ (return to zero phase shift keying) scheme (Non-Patent Document 2) in which an amplitude redundancy is provided, and those that use an ordinary modulation scheme but use an equalizer on the receiver side to exert an anti-multipath property.
For example, spread spectrum schemes include a direct sequence spread spectrum (DSSS) scheme in which an original signal is multiplied by a spread signal having a wider band than that of the original signal, a frequency hopping spread spectrum (FHSS) scheme in which the frequency is hopped over a wide band, and a time hopping spread spectrum (THSS) scheme in which a signal is spread with a wideband impulse.
In order to exert an active path diversity effect by using such a modulation/demodulation scheme with an anti-multipath property, there are conditions as follows with respect to the upper and lower limit for the TDOA (time difference of arrival) between signals. Herein, the minimum and maximum TDOAs with which a path diversity effect can be exerted will be referred to as the “delay resolution” and the “maximum delay”, respectively. The delay resolution and the maximum delay may be determined based on the principle of the modulation/demodulation scheme used, or based on the parameters and/or limitations on implementation of the modulation/demodulation scheme.
For example, with the DSSS scheme, the delay resolution corresponds to the 1-chip length of the spread code, and the maximum delay corresponds to an amount of time less than the spread code length. Therefore, when communicating with the DSSS scheme, it is possible, on the receiver side, to separate a receive signal into delayed wave components and combine them together (RAKE reception) to obtain a path diversity effect as long as the TDOA is greater than or equal to the 1-chip length and less than the spread code length.
With the OFDM scheme, the delayed wave components are absorbed at the guard interval set for the signal, whereby the maximum delay corresponds to the temporal length of the guard interval. Intersymbol interference does not occur if the TDOA between delayed waves is within the guard interval. Moreover, since an error correction operation is normally performed over a plurality of subcarriers, information can be reproduced even if some subcarriers have errors therein due to a multipath distortion. The delay resolution corresponds to a value around the inverse of the frequency bandwidth. Thus, with the OFDM scheme, it is possible to obtain a path diversity effect based on the effect of the guard interval and on the frequency diversity effect provided by scattering pieces of information over a wide frequency band and collecting the pieces together.
Where the PSK-VP scheme or the PSK-RZ scheme, being an anti-multipath modulation scheme, is used, it is possible to exert a path diversity effect and improve the reception characteristics, as compared with an environment where there is no multipath, as long as the delay resolution is greater than or equal to a value that is several times less than the symbol length while the maximum delay is less than the 1-symbol time. Moreover, even with an ordinary single carrier scheme such as the PSK scheme and the QAM scheme, if an equalizer using a tapped delay line is used on the receiver side, it is possible to separate and combine delayed wave components and to exert a path diversity effect with the delay resolution being greater than or equal to the symbol length and the maximum delay being less than or equal to a temporal length determined by the number of taps.
An example of a wireless transmission system that uses such a modulation/demodulation scheme with anti-multipath capabilities to deliberately produce a path diversity effect and to improve the transmission characteristics will now be described.
Patent Document 1 discloses a wireless transmission system using a modulation/demodulation scheme with an anti-multipath property. FIG. 40 is a block diagram showing the wireless transmission system disclosed in Patent Document 1. FIG. 40 only shows the downstream path in which a signal is transmitted from a base station 310 to a mobile station. In FIG. 40, the base station 310 forms a communication area (wireless zone) 300 and communicates with a mobile station 330 within the area using the CDMA (code division multiple access) scheme.
The signal outputted from a wireless device 311 in the base station 310 is transmitted via a transmission antenna 322 to a relay unit 320 and to the mobile station 330. In the relay unit 320, a signal S1 received by a reception antenna 322 is delayed by a delay element 324 and inputted to a combiner 323. A signal S2 received by an antenna 321 is directly inputted to the combiner 323. The combiner 323 combines the signals S1 and S2 together. The combined signal from the combiner 323 is amplified by an amplifier 325 and transmitted to the mobile station 330 via a transmission antenna 326.
The mobile station 330 is a RAKE receiver and receives three signals: a signal delayed by the relay unit, a signal not delayed by the relay unit, and a signal transmitted from the transmitting station. In the relay unit 320, the delay element 324 gives the signal S1 a delay greater than or equal to the code length of the spread code series (chip length), whereby a delay greater than or equal to the chip length is produced between a plurality of signals. Then, the signals are received on the receiver side in RAKE reception, thereby obtaining a path diversity effect and thus improving the transmission characteristics. The wireless transmission system deliberately provides an additional transmission path/delayed wave as described above, aiming at increasing the path diversity effect and improving the transmission characteristics.
Patent Document 1: Japanese Patent No. 2764150
Patent Document 2: Japanese Patent No-2506748
Non-Patent Document 1: H. Takai, “BER Performance of Anti-Multipath Modulation Scheme PSK-VP and its Optimum Phase-Waveform”, IEEE, Trans. Veh. Technol., Vol. VT-42, November 1993, pp. 625-640
Non-Patent Document 2: S. Ariyavisitakul, S. Yoshida, F. Ikegami, K. Tanaka, T. Takeuchi, “A Power-efficient linear digital modulator and its, application to an anti-multipath modulation PSK-RZ scheme”, Proceedings of IEEE Vehicular Technology Conference 1987, June 1987, pp. 66-71
Non-Patent Document 3: S. Ariyavisitakul, S. Yoshida, F. Ikegami, T. Takeuchi, “A Novel Anti-Multipath Modulation Technique DSK”, IEEE Trans. Communication, Vol. COM-35, No. 12, December 1987, pp. 1252-1264
In an anti-multipath modulation/demodulation scheme as described above, there may be cases where the maximum number of branches being effective (hereinafter referred to as the “the maximum number of effective branches”) is limited to a small number, for independent branches that contribute to the path diversity effect, for the following reason. While the maximum number of effective branches that contribute to the path diversity effect is less than or equal to a value obtained by dividing the maximum delay by the delay resolution, this becomes a very small value when the maximum delay is close to the delay resolution.
For example, where the maximum number of effective branches is two, if two waves arrive with a delay therebetween that is equal to the delay resolution with a third wave arriving therebetween, the third wave will be superimposed on both of the first two waves and remain in both waves even after the path separation at the receiver, thereby increasing the inter-branch correlation in path diversity and thus causing deterioration. Where the maximum delay is close to the delay resolution, and the maximum number of effective branches that contribute to the path diversity effect is limited to a small number, the problem is not solved simply by adding a path with a delay. Examples found in the prior art, including Patent Documents 1 and 2, fail to present a method for solving this problem.
A case where the maximum delay is close to the delay resolution and the maximum number of effective branches that contribute to the path diversity effect is limited to a small number will be further described in detail, with respect to various modulation/demodulation schemes.
With the DSSS scheme, where the maximum delay corresponds to a value that is less than the spread code length, if the spread code length becomes short and comes close to the spreading chip length corresponding to the delay resolution, the maximum number of effective branches will be a small number. For example, where the spread code length is a 4-chip length and the spreading factor is four, i.e., one symbol is spread with a 4-chip spread code, the delay resolution is equal to the 1-chip length and the maximum delay is equal to the 3-chip length, whereby the number of branches is about four at best. With the FHSS scheme, the delay resolution corresponds to the spread bandwidth, and the maximum delay is determined by the hop sequence length. Therefore, if the spread bandwidth is narrow and the hop sequence length is short, the maximum number of effective branches is limited to a small number.
Moreover, with the THSS scheme, the delay resolution corresponds to the pulse width and the maximum delay is determined by the pulse sequence length. Therefore, if the pulse width is wide and the pulse sequence length is short, the number of branches is limited to a small number. Similarly, with the OFDM scheme, the delay resolution corresponds to the frequency bandwidth over which subcarriers are distributed, and the maximum delay is determined by the guard interval length. Therefore, if the frequency bandwidth is narrow and the guard interval is short, the maximum number of effective branches is limited to a small number. With the PSK-VP scheme or the PSK-RZ scheme, where the maximum delay cannot in principle exceed the symbol length, the delay resolution is close to the maximum delay in the first place.
This will now be described in detail with respect to the PSK-VP scheme based on the results of a characteristics evaluation.
FIG. 41 shows the bit error rate characteristics relative to the TDOA between two waves in a 2-wave rice model of a quadrature PSK-VP scheme (hereinafter “QPSK-VP scheme”). The horizontal axis represents the TDOA normalized with the symbol length T, and the vertical axis represents the bit error rate. Note that the transmission path is a 2-wave rice fading environment where Eb/No=25 dB. FIG. 41 indicates that in the TDOA range from 0.3 symbol to 0.7 symbol, the path diversity effect provides an active improvement, realizing desirable bit error rates less than or equal to 1E-5. Thus, the delay resolution and the maximum delay with which an active improvement by the path diversity can be obtained are about 0.3 symbol and about 0.7 symbol, respectively.
FIG. 42 shows the bit error rate characteristics for two received waves (two signal-receiving timings) and for three received waves (three signal-receiving timings) in the QPSK-VP scheme, and FIG. 43 shows the temporal relationship between the two waves and the three waves used in FIG. 42. Note that each received wave is a rice fading wave, and the three wave model is a transmission path model where the third wave is inserted at a temporal position in the middle between the two waves. It can be seen from FIG. 42 that the bit error rate deteriorates when the third wave is inserted between the first two waves, as compared with a case where the number of waves to be received is two. This confirms that the third wave in the three-wave model is not separate from the other two waves, thereby giving the same interference or increasing the correlation, thus resulting in a deterioration.
Where an equalizer is used, the delay resolution is determined by the symbol length, and the maximum delay is determined by the tap length of the equalizer filter. Therefore, a case similar to those described above results if the temporal length of the filter tap is shorter than the symbol length. Note that with an equalizer, where the number of taps significantly influences the circuit scale, the maximum delay is in many cases limited due to the circuit scale limitation.
Thus, if the delay resolution, with which delayed wave components can be separated from each other, is significantly close to the maximum delay, the maximum number of effective branches that contribute to the path diversity effect is limited to a small number. Then, carelessly adding a path with a delay will deteriorate the transmission characteristics.
Therefore, an object of the present invention is to provide a wireless transmission system and a wireless transmission method capable of exerting a maximum path diversity effect even if the maximum number of effective branches that contribute to the path diversity effect is limited to a small number, and to provide a wireless station and a transmitting station for use therein.